Chemical, X-ray, electron optical, and infrared analyses have been made on twenty antigorites form the Nishisonogi and Sasaguri areas, northern Kyushu, Japan, along with two antigorites from other localities. The indexed X-ray powder patterns give various supercell A parameters (A = 35.4–47.2Å) as well as varying subcell dimensions (a = 5.42–5.46, b = 9.24–9.26, c = 7.24–7.28Å, (β = 91.3–91.7°). When the ratio of A⁄a is represented by M (M = 6.5–8.7), the electron diffraction patterns can be classified into M = n (n is integer), M = (2n + 1)/2, and M ≠n⁄2 types. The minerals with these different types of M aggregate to form common antigorite specimens. The well-known A = 43Å antigorite belongs to the M = n type. Single crystal X-ray and electron diffraction patterns indicate that the true superstructure periodicity along the X axis of the antigorites giving M = (2n + 1)/2 type patterns, which may contain odd-numbered octahedra in the one alternating-wave, is 2A (corresponding to two waves) and the space lattice is C-centered. This lattice seems to give more reasonable structures at the inversion points of the alternating-waves than hitherto predicted (Kunze, 1961). The M ≠n⁄2 type patterns may be caused by coherent domains with different A parameters of the former two types. The structural formula of antigorite can be given by Mg₆Si₄(1 + 1/2M)^O10(1+1/2M)^(OH)8-2M. Octahedral Mg is substituted by Fe²⁺, and Al or trivalent cations substitute for both the tetrahedral and octahedral cations, although the trivalent cations may be contained more in the octahedral positions for most materials. Larger Fe²⁺ content (FeO 5.5% in max.) tends to bring larger a and b, but smaller c and M(A) parameters. The small c is also produced by relatively large Al contents (Al₂O₃ 4.1% in max.), which supports the presence of tetrahedral Al together with the infrared 3570 cm⁻¹ band. The main OH band at 3685–3674 cm⁻¹ tends to decrease in frequency with increasing Fe content and decreasing M parameter. The correlations of the Fe contents to the a and M parameters and to the main OH band are somewhat different between the Nishisonogi and Sasaguri antigorites, which are discussed in relation to their formation conditions.

View full abstract

The phase equilibria on the join MgSiO₃–MnSiO₃ were determined experimentally in the pressure – temperature range 1–25 kb and 600–1150°C. The following phase assemblages were encountered from MgSiO₃ side to MnSiO₃ component, the single phase of orthopyroxene_SS, orthopyroxene + clinopyroxene, clinopyroxene_SS (kanoite), clinopyroxene + pyroxmangite, pyroxmangite_SS, pyroxmangite + rhodonite, and rhodonite_SS. Below 700°C, olivine is present and the following two fields are encountered; olivine + clinopyroxene + quartz and olivine + quartz + pyroxmangite.
Rhodonite has a limited field (95 mol% MnSiO₃) for all pressures and temperatures. Solubility of MgSiO₃ in pyroxmangite increases linearly with increasing temperature, at least below 10 kb, and that of MnSiO₃ in orthopyroxene increases with increasing pressure. Unit cell constants of the pyroxenes increase linearly with increasing MnSiO₃ content. The clinopyroxene is indexed in the space group P2₁/c, analogous to the natural kanoite. The miscibility gaps observed in our study are consistent with those in natural samples from Tatehira mine, Hokkaido, Japan.

View full abstract

Natronambulite, (Na, Li)(Mn, Ca)₄Si₅O₁₄OH(Na > Li; Mn > Ca) is triclinic, P1 or P-1, a = 7.620, b = 11.762, c = 6.737Å, α = 92.81°, β = 94.55°, γ = 106.87°, Z = 2. The electron microporbe and wet chemical analyses gave SiO₂ 50.39, TiO₂ 0.03, FeO 0.31, MnO 38.94, MgO 1.24, CaO 3.66, Na₂O 3.55, Li₂O 0.43, H₂O⁺ 1.46, H₂O⁻ 0.54, total 100.55%, yielding the empirical formula (Na_0.69Li_0.17)Σ_0.86(Mn_3.32Ca_0.39Mg_0.19 Fe_0.03)Σ_3.93Si_5.07O_14.02(OH)_0.98 (basis O = 15), or ideally (Na, Li)(Mn, Ca)₄Si₅O₁₄OH (Na >Li; Mn > Ca).
It is pinkish orange in colour with a vitreous luster and nearly white streak. H.(Mohs) = 5^1⁄2−6. Density (g/cm³) 3.51 (meas.), 3.50 (calc. using the empirical formula). Cleavage [100] and [100], perfect. Optically it is biaxial and positive, 2V ≈ 45°, r > v, moderate. Refractive indices are: α = 1.706(2), β = 1.710(2), γ = 1.730(5). It is nearly colourless in thin section.
It occurs in a low grade manganese ore from the thermally metamorphosed manganese ore deposit of the Tanohata mine, Iwate Prefecture, Japan, in association with manganoan aegirine, manganoan arfvedsonite, quartz and rhodonite, or with albite, microcline, quartz and serandite.

View full abstract

Johannsenite from the Koryu mine, a gold-silver vein deposit of epithermal type, occurs as a principal constituent of gold-silver quartz veins. It is found as crustified bands in the form of aggregates of very fine crystals, less than 1 μm in length, and sometimes as acicular or fibrous crystals about 10 μm or less in length. Its unitcell parameters are a = 9.875(4), b = 9.044(4), c = 5.274(2)Å, and β = 105.54(9)°. Its chemical composition ranges from 57.7 to 75.6 mole% CaMnSi₂O₆, 14.0 to 23.9 mole% CaMgSi₂O₆, and 10.4 to 18.4 mole% CaFeSi₂O₆.

View full abstract